1. Field of the Invention
The invention relates to parallel detectors of electrons and, more particularly, to an apparatus for attenuating the intensity of an electron beam incident on the detector in an electron energy loss spectrometer.
2. Description of Prior Art
When a beam of high energy electrons traverses a thin sample, the electrons experience various energy losses, typically in the range of 10 to 3000 eV, which are characteristic of the chemical composition of the sample. The energy losses can be analyzed by an electron energy-loss spectrometer. If such a spectrometer is attached to a transmission electron microscope capable of illuminating an extremely small part of a thin sample with an intense electron beam, the combined instrument can analyze the composition of amounts of matter consisting of as little as 10 atoms. Because the irradiating electron beam irreparably damages the thin sample, it is important that the energy loss spectrum be detected with the highest possible efficiency, particularly at energy losses greater than about 100 eV, where the intensity of the electron energy-loss spectrum is usually weak. On the other hand, it is also important that the detector be able to faithfully record the intensity profile of the more intense low energy losses ranging from about 10 eV to 100 eV energy loss, and also of the zero-loss peak, whose intensity is typically 10.sup.6 to 10.sup.9 times higher than the intensity of the highest energy losses of interest. Further, it is desirable that the intense zero loss peak never be allowed to rest upon the detector for an extended time period in order to avoid radiation damage to the detector.
A particularly efficient apparatus for detecting the electron energy-loss spectrum is a detector consisting of a large number of elements which detect a major portion of the spectrum simultaneously. Such detectors are typically called parallel (or multi-channel) detectors of electrons, and electron energy-loss spectrometers employing such detectors are known as parallel-detection electron energy-loss spectrometers. Prior-art parallel detectors used in parallel-detection electron energy-loss spectrometers typically employ a one-dimensional or two-dimensional detector such as a photodiode array, a charge-coupled device array, or a TV camera, and are either directly exposed to the electron beam, or the electron beam is converted into a light image which is optically coupled to the detector. These parallel detectors can typically detect an electron energy-loss spectrum consisting of 512 or 1024 independent channels simultaneously, and can be optimized to detect single incident electrons. However, the dynamic range of the prior-art parallel detectors is typically only 10.sup.2 to 10.sup.5, so that when they are optimized for detecting single electrons, they become hopelessly saturated by the intense zero loss peak.
In order to be able to detect the weak high energy losses as well as the intense zero loss peak, a known parallel detector for an electron energy-loss spectrometer employs two adjacent electron detectors, with one detector optimized for high sensitivity, and the other for ability to handle intense electron beams. The detector also comprises a deflecting dipole which directs the dispersed electron energy-loss spectrum to either one or the other detector according to which part of the spectrum is being studied. Apart from the disadvantage of added complexity of two whole detection systems are compared to one, this design also causes the spectrum to become slightly defocused when deflected from one detector to the other, which necessitates a tedious re-adjustment of focus each time the detectors are switched. In another known parallel detector, an intensifier with variable gain is inserted between an electron scintillator and a light detector, and the gain of the intensifier is adjusted according to the intensity of the part of the electron energy-loss spectrum being studied. This design results in simplified construction and operation, but the insertion of the intensifier typically degrades the resolution of the detector, and non-uniformities in the gain of the intensifier cause the gains of different detector channels to become different from each other.
A further major disadvantage of the known parallel detectors is that when studying the intensity profile of the intense zero loss peak, the detector is continuously exposed to a high flux of oncoming electrons. The accumulated high dose typically causes radiation damage in the detector, requiring a costly replacement of the detector.